Lec 65: Cell Cycle

Question 1

What are the cell cycle phases?

Answer 1

G1 –> S –> G2 –> M

Question 2

Describe G1 Phase

Answer 2

1. cells grow in size and functional output
   
   • mRNAs and protein synthesis
   • organelle expansion
2. some cells remain in G1 for extended period (G0 phase)
3. prepare for S phase
   
   • make materials required for DNA synthesis: enzymes, nucleotides, ATP

Duration: most variable (hours to years)

Question 3

Describe S Phase

Answer 3

1. duplicate each chromosome once
   
   • 2n –> 4n
2. DNA packaging must also be copied: epigenetics

Duration: 10-12 hours

Question 4

Describe G2 Phase

Answer 4

1. cells grow in preparation for division
2. prepare for M phase: make materials for cell division
   
   • tubulin, mitotic spindle proteins, AT

Duration: a few hours

Question 5

Describe M Phase

Answer 5

1. mitosis or karyokinesis
   
   • sister chromatid (DNA) separation
   • prophase, prometaphase, metaphase, anaphase, telophase
2. cytokinesis
   
   • cytoplasmic division

Duration: about 1 hour

Question 6

Describe the mitotic spindle.

Answer 6

• The main machinery in mitosis
• Made of microtubules
  
  • kinetochore microtubules
  • astral microtubules (anchoring)

Question 7

Describe Prophase

Answer 7

• DNA condenses to form compact chromatids
• centrosomes move to poles of nucleus
• microtubules beign to organize into spindle

Question 8

Describe Prometaphase

Answer 8

1. nuclear envelope (membrane) disintegrates
2. spindle microtubules begin attaching to kinetochores on sister chromatids

Question 9

Describe Metaphase

Answer 9

1. chromosomes are aligned at the spindle equator
2. sister chromatids are attached to opposite poles of spindle

Question 10

Describe Anaphase

Answer 10

1. cohesins that hold sister chromatids together are destroyed
2. sister chromatids are separated and move twoards opposite poles by 2 mechanisms
   
   1. kinetochore microtubules shorten
   2. spindle poles move further apart from one another

Question 11

Describe Telophase

Answer 11

1. chromosomes are fully separated to opposite poles
2. nuclear envelope reforms
3. DNA decondenses

Question 12

Describe Cytokinesis

Answer 12

1. accomplished by progressive constriction and final scission of plasma membrane
   
   • contractile ring (inside of cell) of actin and myosin pulls plasma membrane inward as it constricts
2. spindle equator (metaphase plate) determines location
3. active membrane addition is required to accommodate increase in surface area

Question 13

How are cells stimulated to divide?

Answer 13

• mitogens
  
  • erythropoietin
  • antigen
  • cytokines
• environmental stimuli initiates signal transduction cascade (turning on cyclins)

Question 14

What are Cyclins?

Answer 14

proteins expressed at different stages in the cell cycle and direct the events that occur during that stage

Question 15

How do cyclins regulate transit of cell through cell cycle?

Answer 15

• bind w/ cyclin-dependent kinases (Cdk) to phosphorylate targets
• different cyclin-Cdk complexes are activated/ inactivated during each phase of cell cycle
  
  • G1-cyclin
  • S-cyclin: activates DNA synthesis machinery
  • M-cyclin: activates spindle regulators, histones

Question 16

Describe Cell Cycle Checkpoints and what happens if a checkpoint is not satisfied

Answer 16

1. G1 Checkpoint
   
   • is environment favorable?
   • is DNA damaged?
2. G2/M Checkpoint
   
   • is all DNA replicated properly?
   • is DNA damaged?
3. Meta/Anaphase Checkpoint
   
   • are all chromosomes attached to spindle?
4. Check point not satisfied?
   
   • Cdk-inhibitor proteins turn off Cdk-cyclin complex until the problem can be dealth with
     
     • repair or
     • apoptosis

Question 17

Examples of diseases that arise due to cell cycle checkpoint errors/dysfunction?

Answer 17

Cancers

Down Syndrome

Question 18

Describe what occurs when microtubules are poisoned

Answer 18

1. prevents microtubule assembly
2. cell does not make it past metaphase checkpoint
3. causes cell cycle arrest and apoptosis